DESCRIPTION: This application explores stimulation-secretion coupling in the beta islet cells of the pancreas. The investigator has hypothesized that a subpopulation of insulin granules are 'loosely co-localized' with Ca2+ channels in the plasmalemma, and these granules are released rapidly in response to local increases in Ca2+ concentration. The specific aims test four predictions of this co-localization model: 1) release should have a distinctive relationship to total Ca2+ entry that should be independent of the conditions initiating that entry; 2) widely-separated depolarizations should evoke insulin release with distinct latency periods; 3) sustained Ca2+ entry should deplete the co-localized pool of granules, resulting in a period of quiescence while that pool is replenished; and 4) conditions that increase the size of the co-localized pool should permit prolonged bouts of secretion. To evaluate these predictions, the investigator will use instrumentation capable of measuring these parameters of stimulation and secretion simultaneously in single beta cells.